Drilling fluid systems are formulated to have specific properties in order enhance success during petroleum recovery operations. Fluids used during petroleum recovery operations typically are classified according to whether the continuous phase of the fluid comprises water or oil. A water-base fluid comprises water as the continuous phase. An oil-base fluid comprises oil (or organic synthetic) as the continuous phase. An invert emulsion fluid comprises oil or organic synthetic as the continuous phase and an emulsified aqueous phase.
Invert emulsion fluid systems tend to exhibit high performance with regard to shale inhibition, borehole stability, and lubricity. Unfortunately, invert emulsion fluid systems also have a high risk of “loss of circulation.”
“Loss of circulation” generally refers to loss of whole drilling fluid in quantity to the formation. Loss of circulation generally occurs when a drilling fluid system contacts certain subsurface defects during petroleum recovery operations. Examples of such subsurface defects include, but are not necessarily limited to: (1) natural or intrinsic fractures, (2) induced or created fractures; (3) cavernous formations (crevices and channels), and (4) unconsolidated or highly permeable formations (loose gravel and/or sand). Loss of circulation can have very costly and even catastrophic consequences, including shutdown of petroleum recovery operations.
U.S. Pat. No. 6,703,351 describes water base drilling fluid systems comprising polymer latex, which is said to be “capable of providing a deformable latex film or seal on at least a portion of a subterranean formation.” Id. at col. 3, ll. 24-26. In certain circumstances, however, it is desirable to use an oil base drilling fluid system during petroleum recovery operations.
U.S. Publication No. 2004/0132625 describes a latex additive for addition to oil base drilling fluid systems. Unfortunately, the latex additive comprises an aqueous carrier.
The addition to an oil base drilling fluid system of a latex additive comprising an aqueous carrier alters the water-to-oil ratio of the oil base drilling fluid system. In essence, the oil base drilling fluid system becomes an unbalanced invert emulsion drilling fluid system having different rheological properties. For example, plastic viscosity (PV) and/or yield point (YP) tends to be higher in the resulting unbalanced system. See initial and “final” properties of Formula 25 and 26 in Table III, U.S. Publication No. 2004/0132625. An increase in PV and/or YP can increase the equivalent circulating density (“ECD”) of the invert emulsion fluid system, which, in turn, can decrease the ability of the latex additive to reduce loss of circulation.
It is possible to rebalance an unbalanced invert emulsion fluid system. However, an unbalanced invert emulsion fluid system would either have to be rebalanced in the field or transported offsite to be rebalanced. Either option renders the use of latex additive comprising aqueous carrier inefficient.